1. Field of the Invention
The present invention relates to a positioning method of a spot welding robot which has a pair of electrode tips, i.e., a movable electrode tip and an opposition electrode tip opposed to the movable electrode tip, and which sandwiches a work piece to be welded between the pair of electrode tips, and while applying pressure, welds the work piece.
2. Description of Related Art
In spot welding performed by a spot welding robot, in general, welding is performed on a work piece between a pair of opposing electrode tips, i.e., a movable electrode tip and an opposition electrode tip by applying a predetermined pressure to it. Thus, welding is performed on the work piece sandwiched between a pair of electrode tips with no gap therebetween. If there is any gap, sputtering is induced during energization of the electrode tips, leading to degradation of welding quality. The work piece is held at a prescribed position by a jig or the like that is disposed around the robot, and the robot approaches to the work piece so as to sandwich it between a pair of electrode tips.
However, the position of the work piece held by the jig or the like may be altered, or the thickness of the work piece may be changed, or the thickness of the work piece may be varied, and as a result, it may not be possible to sandwich the work piece between a pair of electrode tips without producing a gap. Thus, a gap may be produced between the electrode tips and the work piece. In order to avoid occurrence of such a gap, it is necessary to position the pair of electrode tips of the spot welding robot accurately in the plate-thickness direction of the work piece.
An example of conventional method of positioning a spot welding gun is disclosed in Japanese Patent No. 3337448 (Japanese Patent Publication No. 2001-150150). In this method, a reaction force exerted to a robot due to elastic deformation of a work piece when a pair of electrode tips of a spot welding gun is brought into contact with the work piece is determined based on the variation of current value of a servo motor, and the position of the electrode tips is thereby determined. Thus, the reaction force detected by the servo motor that moves the movable electrode tip and the reaction force detected by the servo motor of the robot that moves the opposition electrode tip are monitored respectively, and a contact state of respective electrode tips with the work piece is thereby determined and positions of respective electrode tips can be thereby determined.
In the above-described method, the reaction force detected by the servo motor of the robot includes frictional force in a robot mechanism. Thus, especially in the case of large spot welding gun having a mechanism of low rigidity, since the reaction force at the time of contact is small, the reaction force may not be detected correctly due to the influence of dynamic friction generated in the detecting operation of the robot. Therefore, a position of a work piece cannot be detected in expected positional accuracy, and the position of the opposition electrode tip may not be detected with sufficient accuracy. Although the influence of dynamic friction can be eliminated or reduced by suppressing the speed of detecting operation of the robot, this may increase the time required for detection of the reaction force, and therefore, may increase the time required for the operation for positioning the spot welding robot.